The density of components mounted on wiring boards is rapidly increasing in response to the further downsizing and advancement of electronic apparatuses. For mobile devices, in particular, there are growing demands for electronic components mounted on thinner wiring boards to occupy a lower profile.
In response to these demands, a thinner electrically insulating base has been developed for the wiring boards used in these types of electronic apparatuses. For example, thinner boards but with the same number of layers are now feasible. A four-layer rigid board of 0.25 mm or thinner and multilayer flexible board about 0.1 mm thick have been put into practical use. For applications which conventionally require six to eight layers in multilayer boards, wiring is housed in fewer layers by the use of high-density wiring boards such as build-up wiring boards. As a result, demands for thinning wiring boards are increasing.
Since the rigidity of these thin boards is lower, the wiring boards are prone to warping. Drying, curing shrinkage, material properties, and rigidity of solder resist formed on the surface of wiring board as a wiring protection layer significantly affect the degree of warping of the wiring board.
Methods of correcting the warping of wiring boards are disclosed in Japanese Patent Unexamined Publication No. H08-45984, Japanese Patent Unexamined Publication No. 2002-83843, and Japanese Patent Unexamined Publication No. 2002-324824, with respect to a tape carrier for TAB (tape automated bonding). In these prior arts, warping is corrected by drying and curing the solder resist under conditions where the wiring board is curved when the solder resist is formed on a film base.
However, in the manufacturing process of wiring boards for board sizes of about 300 mm to 500 mm, curvature cannot be accurately controlled, resulting in difficulty in applying the conventional methods.
A conventional wiring board is described next with reference to FIGS. 6A to 6E.
FIG. 6A is a plan view illustrating the surface of a wiring board sheet. Multiple product portions 1 are provided on the wiring board. Here, the product portion is a unit product area on which an electronic component is mounted to gain a circuit function. It is the portion which will be built into an electronic apparatus.
In general, in addition to product portions 1 on the wiring board, solder resist is formed continuously on the surface and rear face of product portions 1 as a wiring protection layer. A wiring pattern and a solder resist pattern on each product portion 1 are determined based on the product's specifications. Accordingly, the stress in the thickness direction is not balanced locally, and this causes slight warping.
When product portions 1, which contain this slight warping respectively, are provided on the wiring board sheet, each slight warping accumulates, resulting in generating a large warping on a large sheet.
FIG. 6B is a sectional view illustrating warping, taken along line A-A′ in FIG. 6A. In this case, as an example, product portion 1 is warped in a convex manner on the surface.
FIG. 6C is a magnified view of part B in FIG. 6A where product portions 1 are continuously provided. FIG. 6E is a rear view of FIG. 6C. Here, an opening is created in wiring protection layer 2 on the surface of product portion 1, and electronic component connection pad 3 and wiring pattern 5 are exposed from this opening. On the rear face, connection pads 4 aligned in a grid are exposed from wiring protection layer 2.
In this type of thin board, warping occurs to an even more severe extent on product portion 1 due to the difference in opening patterns in wiring protection layers 2 on the surface and rear face. Warping accumulates as a result of continuously disposing product portions 1, as shown in FIG. 6C. FIG. 6D is a sectional view illustrating warping, taken along C-C′ in FIG. 6C. In FIG. 6D, the height at both ends of the cross-section taken along C-C′ in FIG. 6C is leveled for illustrating a schematic sectional view, and thus the drawing shows warping displacement as a conceptual figure, and not the accurate shape.
When electronic components are mounted on a wiring board, they are generally mounted on the wiring board sheet where multiple product portions 1 are provided, rather than mounting each component on independent product portion 1, so as to achieve high productivity in mounting electronic components. As a result, warping generated on the wiring board sheet has been causing failure in transport, low positional accuracy in mounting electronic components, and low mounting reliability in the electronic component mounting process.